1. Field of the Invention
The invention relates to methods and apparatus for accessing the ability of a vehicle emissions control device, such as a lean NOx trap, to releasably store an exhaust gas constituent.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (8), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as HC, CO and NOx, based, for example, on detected values for instantaneous engine speed and engine load (the latter often being inferred, for example, from intake manifold pressure).
To limit the amount of engine-generated constituent gases, such as HC, CO and NOx, that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissions,xe2x80x9d motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NOx xe2x80x9ctrapxe2x80x9d. Both the upstream and downstream catalyst store NOx when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and release previously stored NOx for reduction to harmless gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry.
More specifically, in a typical embodiment, the trap chemically stores NOx during lean-burn operation using alkaline metals, such as barium and/or strontium, in the form of a washcoat. The NOx (NO and NO2) are stored in the trap in the form of barium nitrate, for example. The washcoat also includes precious metals, such as platinum and palladium, which operate to convert NO to NO2 for storage in the trap as a nitrate. The trap""s washcoat typically also includes ceria, whose affinity for oxygen storage is such that, during initial lean engine operation, a quantity of the excess oxygen flowing through the trap is immediately stored in the trap. The amount of stored oxygen is essentially fixed, although it begins to lessen over time due to such factors as increased trap sulfurization (sulfur accumulation) and trap aging.
The trap""s actual capacity to store NOx is finite and, hence, in order to maintain low tailpipe NOx emissions when running xe2x80x9clean,xe2x80x9d the trap must be periodically cleansed or xe2x80x9cpurgedxe2x80x9d of stored NOx. U.S. Pat. No. 5,473,887 teaches the purging of a NOx trap by subjecting the trap to an air-fuel mixture whose air-fuel ratio is rich of stoichiometric, for example, an air-fuel ratio of less than about 13. During the purge event, excess feedgas HC and CO, which are initially consumed in the three-way catalyst to release stored oxygen, ultimately xe2x80x9cbreak throughxe2x80x9d the three-way catalyst and enter the trap, whereupon the trap""s barium nitrate decomposes into NO2 for subsequent conversion by the trap""s precious metals into harmless N2 and O2. The oxygen previously stored in the trap is also released during an initial portion of the purge event after the HC and CO break-through the three-way catalyst.
Because a finite amount of excess fuel is required during the trap purge to release the stored oxygen before stored NOx is released, the prior art has sought to estimate the amount of oxygen stored in the trap by inferring oxygen storage from the time period between combustion, immediately after a rich purge event, of a lean air-fuel mixture within the engine and the switching of the downstream air-fuel ratio from a near-stoichiometric air-fuel ratio to a lean air-fuel ratio, as measured by a downstream oxygen sensor. However, because a significant portion of the time period between the commencement of lean engine operation and the switching of the downstream sensor is caused by movement of the resulting lean exhaust gas through the vehicle""s exhaust system, both upstream and downstream of the trap, the prior art teaches an involved process for separating out the resulting time lags from the overall period, in an attempt to obtain an accurate measure of oxygen storage. Changes in the engine operating condition, i.e., the engine""s speed and load., during this period renders the determination of the trap""s oxygen storage capacity increasingly problematic.
Therefore, a need exists for a method and apparatus for accessing the ability of an emissions control device, such as a lean NOx trap, to releasably store an exhaust gas constituent.
It is an object of the invention to provide a method and apparatus for accessing the ability of a emissions control device for a lean-burn engine to store an exhaust gas constituent.
In accordance with the invention, a method is provided for controlling the operation of an engine of a motor vehicle, wherein the engine generates exhaust gas including an exhaust gas constituent, such as NOx, and wherein exhaust gas is directed through an emissions control device before being exhausted to the atmosphere, whereupon the device stores a quantity of the exhaust gas constituent when the exhaust gas directed through the device is lean of stoichiometry and releasing a previously-stored amount of the exhaust gas constituent when the exhaust gas directed through the device is rich of stoichiometry. Under the invention, the method includes determining that the engine is operating in a first engine operating region immediately following a rich engine operating condition, the rich engine operating condition having been sufficient to release substantially all of the previously-stored amount of the exhaust gas constituent from the device, the first engine operating region being characterized by an exhaust gas space-velocity measure below a predetermined threshold; and, when operating in the first engine operating region, detecting a first air-fuel ratio of the exhaust gas upstream of the device, detecting a second air-fuel ratio of the exhaust gas downstream of the device, and determining a measure representative of an ability of the device when the first air-fuel ratio is lean of stoichiometry and the second air-fuel ratio is not lean of stoichiometry.
In accordance with a feature of the invention, an exemplary embodiment, the exhaust gas space-velocity measure is an air-charge measure, and the threshold for the air-charge measure is no greater than twenty percent of a maximum value for the air charge measure. Most preferably, the air-charge measure threshold is no greater than fifteen percent of a maximum value for the air charge measure.
Other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.